This invention relates to a system for controlling the air/fuel ratio in an internal combustion engine, particularly an automotive engine, by using an oxygen sensor as an exhaust gas sensor to detect actual values of the air/fuel ratio. The oxygen sensor utilizes a solid electrolyte and is a combination of an oxygen concentration cell and an oxygen ion pump cell.
In the current automotive internal combustion engines, it is popular to perform feedback control of the air/fuel ratio by using an oxygen sensor comprising a solid electrolyte cell to estimate actual values of the air/fuel ratio from concentrations of oxygen in the exhaust gas. At starting the engine, however, it is customary to defer the start of the feedback control until completion of the engine warm-up or transiently accelerating operation. In this regard a recent trend is to commence the feedback control of the air/fuel ratio soon after starting the engine with a view to satisfying the demands for better fuel economy and cleaner exhaust gases. Then it becomes necessary to employ an oxygen sensor which is, as an exhaust gas sensor, responsive to changes in the air/fuel ratio in the engine over a fairly wide range including both sub-stoichiometric and super-stoichiometric regions.
Under such circumstances some kinds of oxygen sensors have been developed by modifying the well known oxygen sensor which utilizes an oxygen ion conductive solid electrolyte, such as zirconia, and functions as an oxygen concentration cell. One of the modifications is combining the oxygen concentration cell with another solid electrolyte cell of fundamentally the same construction so as to define a narrow space between the two cells. For example, U.S. Pat. No. 4,450,065 shows an oxygen sensor of this type. In this oxygen sensor the narrow space between the two solid electrolyte cells admits a fraction of the exhaust gas to be measured so that the oxygen concentration cell generates an electromotive force, viz. a voltage, the magnitude of which depends on the partial pressure of oxygen in the gas within the narrow space. The other solid electrolyte cell is used as an oxygen ion pump cell by supplying thereto a DC current, called pumping current, to transfer some oxygen ions from or into the aforementioned space through the solid electrolyte layer of this cell. The pumping current is controlled so as to nullify a difference between the output voltage of the oxygen concentration cell and a reference voltage which corresponds to the target value of the air/fuel ratio in the engine. Therefore, the actual air/fuel ratio can be estimated from the polarity and magnitude of the pumping current needed for equalizing the output voltage and the reference voltage.
For feedback control of an internal combustion engine the oxygen sensor including the oxygen ion pump cell is used in the above described manner on the premise that a definite correlation exists between the adjusted pumping current and the oxygen concentration in the exhaust gas diffused into the measurement space in the oxygen sensor. However, at starting the engine the definite correlation is not established until the solid electrolyte cells in the oxygen sensor are sufficiently heated to reach a fully active state. Even though the oxygen sensor is provided with a heater which can be energized at the time of starting the engine, a length of time elapses before establishment of the expected correlation between the pumping current and the oxygen concentration, and it is difficult to exactly predict the time of establishment of the correlation. This constitutes a difficulty in lessening the delay in starting the feedback control of the air/fuel ratio. If the feedback control is commenced soon after starting the engine without considering the time required for establishment of the expected definite correlation or by hastely and incorrectly judging that the definite correlation has already been established, the exhaust gas composition and the engine operational characteristics become worse. If the feedback control is started with an ample surplus of time for establishment of the definite correlation without trying to ascertain establishment of the correlation, still there is unnecessary delay in the start of the feedback control so that the desire for improvements in the engine performance and exhaust emission cannot fully be met.